1. Field of the Invention
This invention relates to a system for stably controlling an earthquake resistance test or vibration tolerance test device using an electro-hydraulic servosystem.
2. Description of the Prior Art
An example of a vibration test device for testing the earthquake resistance of a structure is disclosed, for example, in U.S. Pat. No. 3,800,588. In the disclosed vibration test device, a structure to be tested is placed on a vibration table which is vibrated to thereby conduct an earthquake resistance test of the structure. A control system of the vibration test device of this type generally constitutes a displacement control system. The configuration and operation of this control system will be briefly described hereunder. The displacement waveform to be reproduced by the vibration table is applied in the form of a voltage signal. This displacement command or input signal is compared with a voltage signal converted from the actual displacement of the vibration table by a detector. The resulting difference, i.e., a control error or deviation is amplified in the form of a current by a servo-amplifier and applied to a servo-valve. In response to this input current, the servo-valve regulates the direction and flow rate of the pressurized fluid supplied to an actuator for driving the vibration table. In this way, the vibration table is driven in a direction to reduce the control error.
In view of the fact that the standards for an earthquake resistance of a structure have been recently established on the basis of the magnitude of acceleration, however, in the vibration or earthquake resistance test device an acceleration control system using an acceleration waveform as a main input signal is used as the control system thereof.
In such a conventional vibration test device having an acceleration control system including an electrohydraulic servo-system, the output waveform of the vibration table is affected by the respective gains of gain-adjusting sections provided in a feedback circuit and an input circuit in the control system. In order to maintain an optimum condition of the output waveform of the vibration table, therefore, it is necessary to adjust the respective values of the gain-adjusting sections. In conventional systems, this adjustment is made manually. This makes an optimum adjustment difficult because of the increased number of parts to be adjusted. Further, the optimum value of each of the gain-adjusting sections varies with the weight of the test object such as a structure placed on the vibration table, and therefore a readjustment is necessary each time a test object of a different weight is placed on the vibration table.